This invention relates to an electric motor for driving a hard disk which is used, for instance, as an external memory unit of a computer.
Heretofore, a device for rotating a hard disk (hereinafter referred to as "a hard disk driver" or "an HDD", when applicable) includes a spindle motor as shown in FIG. 7.
The spindle motor is designed as follows: Its rotary shaft 3 is rotatably supported through rolling bearings 4 an 4 on the inner wall of a cylindrical support 2, which has a flange 1 extended radially outwardly of its periphery. The flange 1 is fixedly mounted on a mounting base board or the like. A hub 5 serving as a rotor is fixedly mounted on one end portion of the rotary shaft 3. A hard disk is fixedly secured to the hub 5.
Each of the bearings 4 and 4 has an inner race 8 which has an inner track 6 in its outer cylindrical wall and is fixedly mounted on the rotary shaft 3, and an outer race 10 which has an outer track 9 in its inner cylindrical wall and is fitted in the inner wall of the support 2, and a plurality of rolling elements 7.
An armature winding 11 is fixedly mounted on the outer wall of the support 2. Rotor magnets 13 are fixedly mounted on the inner wall of the cylindrical part 12 of the hub 5 in such a manner as to confront the outer cylindrical wall of the armature winding 11. Thus, by applying current to the armature winding 11, the hub 5 is rotated around the central axis of the rotary shaft 3.
However, the hard disk driving motor thus constructed suffers from the following difficulty:
As was described above, in order to rotatably support the rotary shaft 3 inside the armature winding 11, the bearings 4 and 4 each comprising the inner race 8, the outer race 10, and the rolling elements 7 are fitted in the cylindrical support 2, and the rotary shaft 3 is fitted in the bearings 4 and 4.
Hence, unavoidably the width T of the gaps 14, which are formed between the cylindrical surface of the rotary shaft 3 and the inner wall of the support to receive the bearings 4 and 4, is relatively large, and accordingly the outside diameter R of the spindle motor is relatively large.
On the other hand, there is a tendency for a hard disk driver installation space to be extremely decreased for instance because of the popularization of lap-top type OA (office automation) equipment. Hence, it is not desirable that the outside diameter D of the spindle motor is increased even slightly because of the above-described reason.